I had this problem last year when I moved from an Nv 8800 to an Ati 4890 on Vista.
I tried lots of things and I hate to say this but the only thing that fixed it for me was a clean OS install of Win7.
The only rational explanation I could think of was an Nvidia driver or codec somewhere that did not get removed in the uninstall and upgrade process, perhaps it wasnt registered correctly with Vista.
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